Device and method for controlling tilt servo

ABSTRACT

A tilt servo control device of an apparatus for recording information on and reproducing information from an optical recording medium, the apparatus comprising an optical system for leading a laser beam emitted from a light source to a recording surface of the optical recording medium and a laser beam reflected by the recording surface of the recording medium to a split type photo detector and a read signal generator for generating a read signal in accordance with an output signal of the photo detector. The tilt servo control device generates a push-pull component signal including a differential component of output signals of respective split portions of the photo detector, generates a tilt error signal representing a tilt angle at a position of the laser beam irradiating the recording surface and an optical axis of the laser beam based on the push-pull component signal, and drives a tilt angle adjuster for adjusting the tilt angle so as to reduce the tilt error signal.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a tilt servo control device and a tiltservo control method for compensating the tilt angle of a recordingsurface of an optical recording medium as defined by the angle betweenthe normal to the recording surface of the recording medium and theoptical axis of a light beam irradiating the recording surface.

2. Description of the Related Background Art

In order to correctly read information recorded on an optical disk suchas a DVD (Digital Versatile Disk), it is necessary to irradiate areading light beam perpendicularly relative to a recording surface ofthe optical disk. If the optical disk has a warp, or errors in themechanical system are large, it is impossible to irradiate the readinglight beam perpendicular to the recording surface of the optical disk,thereby deteriorating reading accuracy for the recorded information.

In view of this problem, information reproducing apparatus forreproducing information recorded on an optical disk are normallyprovided with a tilt servo control device for detecting the tilt betweena pickup as information reading means of the apparatus and the opticaldisk and compensate the tilt by adjusting the direction of the pickup inaccordance with the detected tilt or by providing a signal read out bythe pickup with a tilt correcting process corresponding to the detectedtilt, so as to suppress the deterioration in the information readingaccuracy.

Further, in order to compensate the tilt, a tilt servo control deviceprovided with a liquid crystal panel inserted into the optical axis isknown (For example, Japanese Patent Laid-Open Publication No. Hei.11-3531). In the case of the device, the liquid crystal panel is dividedinto a plurality of regions, and a phase difference is provided to alight beam passing through each of the regions of the liquid crystalpanel so as to maximize the level of a signal (RF signal) read out froma recording medium by a pickup.

In the case of a writable optical disk such as a DVD-R, a DVD-RW and aDVD-RAM, when no information is written in the writable optical disk, nosignal can be read from the unwritten optical disk. Therefore, there isa problem in that it is not possible to properly perform the tilt servocontrol device for the unwritten optical disk.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a tiltservo control device and a tilt servo control method that can properlyperform tilt servo control for an optical recording medium into whichany information is not written yet.

The present invention provides a tilt servo control device of aninformation recording/reproducing apparatus for recording information onand reproducing information from an optical recording medium, theapparatus comprising an optical system for leading a laser beam emittedfrom a light source to a recording surface of the optical recordingmedium and a laser beam reflected by the recording surface of therecording medium to a split type photo detector and a read signalgenerator for generating a read signal in accordance with an outputsignal of the photo detector. The device comprises a push-pull componentgenerator for generating a push-pull component signal including adifferential component of output signals of respective split portions ofthe photo detector, a tilt error signal generator for generating a tilterror signal representing a tilt angle between a normal to the recordingsurface of the optical recording medium at a position of the laser beamirradiating the recording surface and an optical axis of the laser beambased on the push-pull component signal, a tilt angle adjuster foradjusting the tilt angle, and a driver for driving the tilt angleadjuster so as to reduce the tilt error signal.

The present invention provides a tilt servo control method of aninformation recording/reproducing apparatus for recording information onand reproducing information from an optical recording medium, theapparatus comprising an optical system for leading a laser beam emittedfrom a light source to a recording surface of the optical recordingmedium and a laser beam reflected by the recording surface of therecording medium to a split type photo detector and a read signalgenerator for generating a read signal in accordance with an outputsignal of the photo detector. The method comprises the steps ofgenerating a push-pull component signal including a differentialcomponent of output signals of respective split portions of the photodetector, generating a tilt error signal representing a tilt anglebetween a normal to the recording surface of the optical recordingmedium at a position of the laser beam irradiating the recording surfaceand an optical axis of the laser beam based on the push-pull componentsignal, and driving a tilt angle adjuster for adjusting the tilt angleso as to reduce the tilt error signal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a disk player provided with a tilt servocontrol device according to the present invention.

FIG. 2 is a schematic diagram illustrating the recording format of aDVD-RW.

FIG. 3 is a schematic diagram illustrating the recording format of ageneral purpose DVD-R.

FIG. 4 is a diagram illustrating a physical structure of a recordingsurface of a DVD-RW or DVD-R.

FIG. 5 is a graph illustrating the waveform of a groove wobble signalincluding LLP components.

FIG. 6 is a schematic diagram illustrating recording tracks of aDVD-RAM.

FIG. 7 is an enlarged view of a recording surface of the DVD-RAM of FIG.6.

FIG. 8 is a schematic diagram illustrating the recording format of theheader section of each sector located at a changeover point of theDVD-RAM of FIG. 6.

FIG. 9 is a schematic diagram illustrating the recording format of theheader section of each sector without a changeover point of the DVD-RAMof FIG. 6.

FIG. 10 is a schematic diagram illustrating VFO data of a header sectionand a data section.

FIG. 11 is a schematic diagram illustrating different regions of aliquid crystal panel arranged in a pickup of FIG. 1.

FIG. 12 is a structural diagram schematically illustrating the opticalsystem of the pickup of FIG. 1.

FIG. 13 is a schematic diagram illustrating beam spots formed on anoptical disk.

FIG. 14 is a schematic block diagram illustrating a circuit including apickup, a tracking servo circuit and a tilt servo circuit.

FIG. 15 is a schematic block diagram of a focusing servo circuit and aspindle servo circuit.

FIG. 16 is a schematic block diagram of a tilt servo drive system.

FIG. 17 is a flow chart illustrating the operation of a disk typediscriminating circuit.

FIG. 18 is the remaining portion of the flow chart of FIG. 17.

FIG. 19 is a graph illustrating the S-characteristics of a CD type diskand a DVD type disk.

FIG. 20 is a flow chart of the tilt servo control operation of a firsttilt servo section for a general purpose DVD-R or a DVD-RW.

FIG. 21 is a flow chart of a tilt servo control operation of the firsttilt servo section for an authoring purpose DVD-R.

FIG. 22 is a flow chart of an alternative tilt servo control operationof the first tilt servo section for an authoring purpose DVD-R.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention will now be described in detailhereinafter with reference to the accompanying drawings.

FIG. 1 is a schematic block diagram of an optical disk player providedwith a tilt servo control device according to the present invention. Inthe illustrated optical disk player, a pickup 10 irradiates a laser beamonto an optical disk 12 and receives a laser beam reflected by theoptical disk 12. Then, the pickup 10 generates a signal corresponding tothe intensity of the received laser beam. The optical disk 12 is drivento rotate by a motor 14. The optical disk 12 may be a DVD type disk suchas a DVD-ROM, a DVD-R, a DVD-RAM and a DVD-RW or a CD type disk such asa CD-ROM and a CD-R. The DVD-RAM has two types, a one-recording-layerdisk and a two-recording-layer disk.

Referring to FIG. 2, a DVD-RW has data structure including a PCA (powercalibration area), an RMA (recording management area), a lead-in area, adata area and a lead-out area arranged in the mentioned order from theinner periphery to the outer periphery of the disk. The PCA is an areato be used for a test writing operation so as to determine a recordingpower level of the laser beam and the RMA is an area where managementinformation on the current recording operation is written. The lead-inarea contains an embossed section comprising phase pits formed on thedisk in advance. The embossed section stores information on prohibitionof copying.

A DVD-R may be a general purpose disk or an authoring purpose disk.Referring to FIG. 3, a general purpose DVD-R has a data structureincluding a PCA, an RMA, a lead-in area, a data area and a lead-out areaarranged in the mentioned order from the inner periphery to the outerperiphery of the disk. The lead-in area contains in part thereof apre-write section that is located at a position exactly corresponding tothat of the embossed section and used to store information just like theembossed section. On the other hand, an authoring purpose DVD-R does nothave any section that corresponds to the embossed section in FIG. 2 orthe pre-write section in FIG. 3.

FIG. 4 is a schematic perspective partial view of the recording surfaceof DVD-RW or DVD-R, illustrating its physical structure. Referring toFIG. 4, the recording surface is realized by combining a groove wobblesystem of wobbled grooves 103, formed between lands 102 on a disksubstrate 103, operating as recording tracks and land pre-pits (LPPs)104 formed in the lands 102 to link adjacent grooves 103. Information isrecorded only in grooves 103. The recording surface is covered by aprotection layer 105 typically made of polycarbonate. The signalsobtained by the groove wobble system are mainly used to control therotation motion of the disk during a recording operation and generaterecording master clock for recording operations, whereas the LPPs areused for determining the recording position accurately on a bit by bitbasis and also for obtaining various pieces of information on the disksuch as pre-addresses. For example, as will be described hereinafter,the low frequency zone component of the output signal (push-pull signal)of multiplier 44 is the signal that is obtained by the groove wobblesystem (groove wobble signal), showing a sinusoidal waveform asillustrated in FIG. 5. The narrow width pulses produced in parts of thesinusoidal waveform correspond to LPP components that can be detected bycomparing the level of the signal with a threshold value TH.

In the case of a DVD-RAM, helical tracks are formed on the disk in amanner as schematically shown in FIG. 6. FIG. 7 is an enlarged schematicperspective partial view of the recording surface of the DVD-RAM of FIG.6. The tracks are wobbled as shown in FIG. 7. Lands 131 and grooves 132are arranged alternately for the tracks. Changeover points 133 from thelands 131 to the respective grooves 132 are located on a same radialline. Data are recorded on the tracks on a sector by sector basis. Inother words, a sector provides a unit for recording data. Each sectorcomprises a header section and a data section. The header sectioncomprises physically embossed pits 134 that are displaced from thecenter line of the track and located near the boundary with the adjacenttrack. The data section stores data in the form of phase change pitsthat are formed by producing changes in the reflectance in therespective areas of the surface of the disk hit by a laser beam. Morespecifically, the changes are produced as a result of phase changes. Thelevel of the signal read from the disk will be affected by the recordingprinciple and the recording conditions of the signal.

FIG. 8 is a schematic illustration of the recording format of the headersection of a sector located at a changeover point 133. FIG. 9 is aschematic illustration of the header section of a sector not related toany changeover point. Referring to FIGS. 8 and 9, the both sectorscomprise a header section having four header fields, a mirror surfacesection located immediately after the header section and a data sectionhaving a land or groove structure as described above. An embossed pit isformed in each of the header fields although not shown in FIGS. 8 and 9.The leading two header fields, or the first and second header fields, ofthe four header fields of each land sector are displaced radiallyinwardly from the center line of the track (by a half of the trackpitch), whereas the remaining two header fields, or the third and fourthheader fields, of the land sector are displaced radially outwardly fromthe center line of the track (by a half of the track pitch). The fourheader fields of each groove sector are mirror images of the headerfields of the corresponding land sector.

A fixed data section referred to as VFO (variable frequency oscillator)is arranged in each of the header section and the data section of eachsector as shown in FIG. 10. The VFO data of each VFO shows apredetermined data pattern of repeated “0s” and “1s” for every 4T (Trepresenting the bit interval of successive information data). A VFO isarranged at the leading end of each of the first through fourth headerfields of the header section in each sector. First VFO data of the firstfield and the third field have a length equal to 576 clocks, whereassecond VFO data of the second field and the fourth field have a lengthequal to 128 clocks. A VFO data having a length equal to 560 clocks isarranged in the data section. For instance, a DVD-RAM contains these VFOdata as part of fixed data without fail.

A liquid crystal panel 13 is arranged on the optical axis of the beam oflight in the pickup 10 for the purpose of correcting aberrations inradial directions of the disk in order to make it possible to correctthe wave front aberrations of the optical system. The liquid crystalpanel 13 is typically divided into three regions 13 a through 13 c alonga radial direction as shown in FIG. 11. These regions include an innerperipheral side region, an intermediary region and an outer peripheralside region. The three regions 13 a through 13 c are individually andvariably controlled by the respective drive voltages output from a tiltservo circuit 51 by way of a drive circuit 28 as will be describedhereinafter. With this arrangement, the phase difference of lightpassing through each of the regions 13 a through 13 c can be changedindividually so that the wave front aberrations such as coma that areproduced by a tilt of the disk in a radial direction thereof may becorrected.

As shown in FIG. 12, the optical system of the pickup 10 comprises asemiconductor laser element 21 for emitting a laser beam with awavelength of 650 nm for DVDs and a semiconductor laser element 22 foremitting a laser beam with a wavelength of 780 nm for CDs. Thesemiconductor laser elements 21, 22 are arranged in such a way that thecenter lines of the laser beams emitted from them are perpendicularrelative to each other. A conflux prism 23 is arranged across the laserbeams emitted from the semiconductor laser elements 21, 22 so that thelaser beam emitted from the semiconductor laser element 21 istransmitted through the conflux prism 23 whereas the laser beam emittedfrom the semiconductor laser element 22 is reflected by the confluxprism 23 and made to leave the latter in a direction same as that of thelaser beam emitted from the semiconductor laser 21.

The semiconductor laser element 21 is driven by a drive circuit 18whereas the semiconductor laser element 22 is driven by another drivecircuit 19.

The laser beams coming from the conflux prism 23 are made to get to apolarization beam splitter 26 provided with a polarizer panel 26 a byway of a collimator lens 24 and a grating 25. The grating 25 is arrangedto divide the laser beams into a plurality of fluxes (flux of 0 order,those of ±1 order). In other words, a main beam and a pair of sub beamsare formed by it. The polarization beam splitter 26 allows most (e.g.,90%) of the laser beam entering it to pass through it and the polarizerpanel 26 a transforms the linear polarization of the passing laser beaminto circular polarization.

The laser beam that is allowed to pass through the polarization beamsplitter 26 having the polarizer panel 26 a then gets to the disk 12 byway of the liquid crystal panel 13 and the objective lens 27 thereof andbecomes reflected by the recording surface of the disk 12. The laserbeam reflected by the recording surface of the disk 12 then returns tothe polarization beam splitter 26 by way of the liquid crystal panel 13and the polarizer panel 26 a. The polarizer panel 26 a transforms thecircular polarization of the reflected and returned laser beam intolinear polarization. The polarization beam splitter 26 reflects thereturned laser beam by means of its polarizing/splitting plane 26 b. Thereflected laser beam then gets to the light receiving plane of photodetector 30 by way of condenser lens 28 and multi lens 29.

The pickup 10 further comprises an actuator 34 having a focusing sectionfor moving the objective lens 27 along the optical axis and a trackingsection for moving the objective lens 27 in a radial direction of thedisk that is perpendicular to the optical axis.

As shown in FIG. 13, spots MS, SS1, SS2 are formed on the disk 12 by themain beam and the sub beams of the laser beam coming from the pickup 10.Note that the two sub beam spots SS1, SS2 are displaced from the mainbeam spot MS in a radial direction of the disk by a half of the trackpitch P. This is because the differential push-pull method is used fordetecting tracking errors.

As shown in FIGS. 13 and 14, the photo detector 30 comprises a lightreceiving section 31 for receiving a main beam and a pair of lightreceiving sections 32, 33 for receiving sub beams arranged at oppositesides of the light receiving section 31. The light receiving surface ofthe light receiving section 31 is divided into four areas to producephoto detecting elements 31 a through 31 d, whereas the light receivingsurfaces of the light receiving sections 32, 33 are divided into twoareas to produce photo detecting elements 32 a, 32 b and 33 a, 33 b.

As shown in FIG. 14, the pickup 10 comprises an adder 35 for adding theoutput signal a of the photo detecting element 31 a and the outputsignal c of the photo detecting element 31 c, another adder 36 foradding the output signal b of the photo detecting element 31 b and theoutput signal d of the photo detecting element 31 d and still anotheradder 37 for outputting an RF signal (read signal) by adding the outputsignals of the adders 35, 36. The output of the adder 37 is connected tothe tilt servo circuit 51 and the disk type discriminating circuit 52 ofa servo circuit 5 as well as to a reproduction processing section (notshown). The outputs of the adders 35, 36 are connected to a focusingservo circuit 53.

As shown in FIG. 15, the focusing servo circuit 53 comprises anamplifier 61 for amplifying the output signal of the adder 35, anotheramplifier 62 for amplifying the output signal of the adder 36, asubtracter 63 for generating a focus error signal FE by subtracting theoutput signal of the amplifier 62 from the output signal of theamplifier 61, an equalizer 64 for generating a focus drive signal FD inaccordance with the focus error signal FE that is the output signal ofthe subtracter 63, a switch 65, still another adder 66 and a voltageapplication circuit 67 for generating a variable voltage. The focusdrive signal FD output from the equalizer 64 is fed to drive circuit 68by way of the switch 65 and the adder 66 when the switch 65 is on. Thedrive circuit 68 drives the focusing section of the actuator 34 inaccordance with the focus drive signal FD. The on/off operations of theswitch 65 are controlled by the system control circuit 1. The voltageapplication circuit 67 generates a variable voltage in accordance withthe focus jump command from the system control circuit 6. As a variablevoltage is generated by the voltage application circuit 67, it is addedby the adder 66 and the output signal of the adder 66 become the focusdrive signal FD.

The pickup 10 additionally comprises an adder 38 for adding the outputsignal a of the photo detecting element 31 a and the output signal d ofthe photo detecting element 31 d, another adder 39 for adding the outputsignal b of the photo detecting element 31 b and the output signal c ofthe photo detecting element 31 c and a subtracter 40 for subtracting theoutput signal of the adder 39 from that of the adder 38. The output ofthe subtracter 40 is connected to the disk type discriminating circuit52 and the tracking servo circuit 54 of the servo circuit 5. The outputsignal of the subtracter 40 is a push-pull signal.

As shown in FIG. 14, the pickup 10 further comprises a subtracter 41 forsubtracting the output signal of the photo detecting element 32 b fromthe output signal of the photo detecting element 32 a and anothersubtracter 42 for subtracting the output signal of the photo detectingelement 33 b from the output signal of the photo detecting element 33 a.The outputs of the subtracters 41, 42 are connected to the trackingservo circuit 54.

As shown in FIG. 14, the tracking servo circuit 54 comprises an adderfor adding the output signals of the subtracters 41, 42, a multiplier 44for multiplying the output signal of the adder 43 by a coefficient α, asubtracter 45 for generating a differential push-pull tracking errorsignal TE by subtracting the output signal of the multiplier 44 from theoutput signal (push-pull signal) of the subtracter 40, an equalizer 46for generating a tracking drive signal TD in accordance with thetracking error signal TE output from the subtracter 45 and a switch 47.

The coefficient α of the adder 44 is determined in such a way that theoutput signal (push-pull of the main beam) of the subtracter 40 and theoutput signal (push-pull of the sub beams) of the multiplier 44 are madesubstantially equal to each other. Note that these two output signalshave respective AC components whose polarities are opposite to eachother and respective DC components that show a same polarity.

The equalizer 46 generates the tracking drive signal TD so as to reducethe tracking error signal TE. The tracking drive signal TD output fromthe equalizer 46 is fed to the drive circuit 48 by way of the switch 47when the switch 47 is on. The drive circuit 48 drives the trackingsection of the actuator 34 in accordance with the tracking drive signalTD. The on/off operations of the switch 47 are controlled by the systemcontrol circuit 1.

The tilt servo circuit 51 generates a tilt drive signal TID for drivingthe above described liquid crystal panel 13 and is provided with threetilt servo sections, or the first through third tilt servo sections,that are adapted to selectively operate depending on the type of thedisk 12. The first tilt servo section is used for DVD-Rs and DVD-RWsand, as shown in FIG. 14, comprises an adder 69, a low pass filter 73, achangeover switch 70, a reference memory 71, a subtracter 72, an A/Dconverter 74 and a tilt correction ROM 75. The adder 69 adds the outputsignal of the subtracter 40 in the pickup 10 and that of the multiplier44 in the tracking servo circuit 54.

Since the coefficient α of the multiplier 44 is determined in a manneras described above, the push-pull component of the output of the adder69 is cancelled to leave only the DC component. The output signal of theadder 69 is smoothed as it is input to the low pass filter 73 so thatthe eccentric component of the disk will be removed. The output signalof the low pass filter 73 is supplied to the reference memory 71 by wayof the changeover switch 70 to represents a state involving no tiltbefore an information recording operation and the reference memory 71stores the supplied signal level as reference signal. The changeoverswitch 70 relays the output signal of the low pass filter 73 to thesubtracter 72 during a recording operation. During a recordingoperation, the output signal of the low pass filter 73 is a push-pullsignal containing an offset component that varies as a function of thetilt of the disk 12, or a push-pull offset signal. The subtracter 72generates a first tilt error signal representing the tilt of the disk 12by subtracting the reference signal stored in the reference memory 71from the output signal of the adder 69.

During a recording operation, the output signal of the multiplier 44 andthat of the subtracter 40 are tracking error signals containing anoffset component that represents the tilt of the disk 12. The signalobtained by removing the offset component from the signal produced asthe sum of the two signals by means of the adder 69 is stored in thereference memory 71 as reference signal. Thus, the signal output fromthe subtracter 72 as a result of subtracting the reference signal fromthe output signal of the adder 69 contains only the offset component sothat it is possible to provide the first tilt error signal.

The first tilt error signal is digitized by the A/D converter 74 andthen fed to the tilt correction ROM 75, which stores a plurality of tiltcorrection values and output three correction values stored at therespective addresses specified by the first tilt error signal. Thesethree correction values correspond respectively to the three regions 13a through 13 c of the liquid crystal panel 13.

While the low pass filter 73 is provided in this embodiment, it may beomitted and the first tilt error signal may be obtained directly fromthe subtracter 72 when the eccentric component of the disk is small.

Additionally, while the output signal of the subtracter 40 and that ofthe multiplier 44 are added by the adder 70 in this embodiment, it mayalternatively be so arranged that only either the output signal (mainpush-pull signal) of the subtracter 40 or the output signal (subpush-pull signal) of the multiplier 44 is smoothed by the low passfilter and fed to the reference memory 71 by way of the changeoverswitch 70.

As shown in FIG. 14, the second tilt servo section that is used forDVD-RAMs comprises a delay circuit 76, a subtracter 77, a low passfilter 78, an A/D converter 79 and a tilt correction ROM 80. The delaycircuit 76 delays the RF signal output form the adder 37 and supplies itto the subtracter 77. The RF signal corresponds to the first VFO data(of 64 bytes) in the first and third header fields of the abovedescribed header format of DVD-RAM. The delay time of the delay circuit76 corresponds to the time necessary for passing through the firstheader field and the second header filed of the header section. Thus,while the first VFO data of the first header field is fed from the delaycircuit 76 to one of the input terminals (non-inversion input terminal)of the subtracter 77, the first VFO data of the third header field isfed to the other input terminal (inversion input terminal) of thesubtracter 77. The subtracter 77 subtracts the first VFO data of thethird head field from the first VFO data of the first header field andsupplies a signal representing the different to the low pass filter 78.The low pass filter 78 smooths the signal obtained by the subtractionand generates a second tilt error signal. The second tilt error signalis digitized by the A/D converter 79 and then fed to the tilt correctionROM 80, which stores a plurality of tilt correction values and outputthree correction values stored at the respective addresses specified bythe second tilt error signal. These three correction values output fromthe tilt correction ROM 80 correspond respectively to the three regions13 a through 13 c of the liquid crystal panel 13. Japanese PatentLaid-Open Publication No. 2000-137923 describes the generation of tilterror signal of a tilt servo section to be used for DVD-RAMs in greaterdetail.

The third tilt servo section for DVD-ROMs uses a so-called hill climbingmethod for the purpose of generating correction values in order tomaximize the RF signal. As shown in FIG. 14, it comprises an RFamplitude level detector 81, an L register 82, an H register 83, acomparator 84, an up-down counter 85, an adder and a tilt correction ROM87. The RF amplitude level detector 81 detects the amplitude level ofthe RF signal. The L register 82 holds the amplitude level of the RFsignal obtained by using the current output value of the up-down counter85 as address value for the tilt correction ROM 87. The H register 83holds the amplitude level of the RF signal obtained by adding the outputvalue of the up-down counter 85 and 1 by means of the adder 86 and usingthe sum as address value for the tilt correction ROM 87. The adder 86performs the addition of 1 in response to the command from the systemcontrol circuit 1. The comparator 84 compares the amplitude level heldin the L register 82 and the one held in the H register 83. If the valueheld in the L register 82 is found to be larger than the value held inthe H register 83 as a result of the comparison made by the comparator84, the up-down counter 85 counts up by 1. If, on the other hand, thevalue held in the L register 82 is found to be smaller than the valueheld in the H register 83 as a result of the comparison made by thecomparator 84, the up-down counter 85 counts down by 1. The tiltcorrection ROM 87 outputs the correction values stored at the respectiveaddresses specified by the output value of the adder 86. The threecorrection values output from the tilt correction ROM 87 correspondrespectively to the three regions 13 a through 13 c of the liquidcrystal panel 13.

As a tilt servo start command is issued to the above described tiltservo section for DVD-ROMs, the current output value of the up-downcounter 85 is set in the tilt correction ROM 87 by way of the adder 86as address value. Then, as a result, the liquid crystal panel 13 isdriven by way of a servo drive system, which will be describedhereinafter. The amplitude level of the RF signal detected by the RFamplitude level detector 81 is held by the L register 82. Then, 1 isadded to the current output value of the up-down counter 85 by the adder86 and the sum is set in the tilt correction ROM 87 as address value andused to drive the liquid crystal panel 13 by way of the tilt servo drivesystem. The amplitude level of the RF signal detected by the RFamplitude level detector 81 is held by the H register 83. The comparator84 compares the value held by the L register 82 with the value held bythe H register 83 and counts up 1 by when the value held by the Lregister 82 is larger than the value held by the H register 83, whereasit counts down by 1 when the value held by the L register 82 is smallerthan the value held by the H register 83. Then, the above operation isrepeated. Japanese Patent Laid-Open Publication No. Hei. 11-3531describes such a tilt servo section for DVD-ROMs in greater detail.

The outputs of the three tilt correction ROMs 75, 80, 87 are connectedto selector 88 and then to tilt servo drive system comprising registers96 a through 96 c, PWM sections 97 a through 97 c and a drive circuit28. The selector 88, the registers 96 a through 96 c and the PWMsections 97 a through 97 c are contained in the servo circuit 5. Theselector 88 relays one of the corrected output values of the tiltcorrection ROMs 75, 80, 87. The output of the selector 88 is connectedto the registers 96 a through 96 c for holding a corrected value. Theoutputs of the registers 96 a through 96 c are connected to therespective PWM (pulse width modulator) sections 97 a through 97 c. ThePWM sections 97 a through 97 c perform pulse-width modulates inaccordance with the respective output values of the registers 96 athrough 96 c and supply the respective modulated signals to the drivecircuit 28 as tilt drive signals TID.

Regardless which one of the tilt correction ROMs 75, 80, 87 supplies acorrected value to the registers 96 a through 96 c of the tilt servodrive system by way of the selector 88, the corrected value held by theregister 96 a and the one held by the register 96 c are symmetricalrelative to the corrected value held by the register 96 b. A drivesignal having a pulse width corresponding to the corrected value held bythe register 96 a is generated by the PWM section 97 a. Similarly, adrive signal having a pulse width corresponding to the corrected valueheld by the register 96 b is generated by the PWM section 97 b.Likewise, a drive signal having a pulse width corresponding to thecorrected value held by the register 96 c is generated by the PWMsection 97 c. The drive circuit 28 applies voltages respectively to theregions 13 a through 13 c in accordance with the respective levels ofthe corresponding drive signals. As a result of the voltage application,a light path difference Δn·d (Δn is the variation of the refractiveindex and d is the liquid crystal cell thickness) is produced by thedouble refraction effect of liquid crystal molecules in the regions 13 athrough 13 c. If the wavelength of a light beam passing through theliquid crystal is λ, a phase difference of Δn·d (2π/λ) is given to thelight beam. Thus, it is now possible to correct the aberrationsincluding the coma generated in a radial direction of the disk due tothe tile of the disk.

As shown in FIG. 15, the servo circuit 5 has a spindle servo circuit 93comprising a rotational speed detector 89, a rotational speed errorgenerator 90, an equalizer 91 and a switch 92. A frequency signal FG isfed to the rotational speed detector 89. A frequency signal FG is an ACsignal representing the current revolution frequency of the spindlemotor 14 for driving the optical disk 12 to rotate by way of a turntable. The rotational speed detector 89 generates a rotational speedsignal indicating the rotational speed of the spindle motor thatcorresponds to the frequency signal FG and supplies it to the systemcontrol circuit 1 and also to the rotational speed error generatingsection 90. The rotational speed error generating section 90 generates arotational speed error signal indicating the difference between therotational speed signal and the reference rotational speed signal fedfrom the system control circuit 1 and supplies it to the equalizer 91.In response to the rotational speed error signal, the equalizer 91generates a spindle drive signal SPD, which is fed to the spindle motor14 by way of the drive circuit 83 when the switch 92 is on. The spindlemotor 14 drives the optical disk 12 to rotate with the rotational speedindicated by the spindle drive signal SPD. The AC generator (not shown)arranged in the spindle motor 14 supplies the frequency signal FGrepresenting the current revolution frequency to the servo circuit 5.Thus, with the above-described arrangement of the spindle servo system,the spindle motor 14 is driven to rotate with the rotational speedindicated by the reference rotational speed signal fed from the systemcontrol circuit 1.

Although not shown, the servo circuit 5 generates a slider drive signalSD on the basis of the above tracking error signal TE and supplies it tothe slider 100 by way of the drive circuit 8. As a result, the slider100 moves the pickup 10 in a radial direction of the disk with arotational speed corresponding to the drive current based on the sliderdrive signal SD.

The disk type discriminating circuit 52 determines the type of the disk12 mounted on the turn table. As pointed out earlier, the disks 12 maybe a CD type disk such as CD, CD-ROM or CD-R or a DVD type disk such asDVD, DVD-ROM, DVD-R, DVD-RAM or DVD-RW.

Now, the operation of the disk discriminating circuit 52 will bedescribed by referring to the flow charts of FIGS. 17 and 18.

Firstly, referring to FIG. 17, the disk discriminating circuit 52determines if a recording command is issued from the system controlcircuit 1 or not (Step S1). If it is determined that a recording commandis issued, it determines if the disk 12 is of the CD type or of the DVDtype (Step S2). A CD type disk may be a CD, a CD-ROM or a CD-R. A DVDtype disk may be a DVD, a DVD-ROM, a DVD-R, a DVD-RAM or a DVD-RW. Forthe operation of Step S2, a laser beam having a wavelength of 780 nm isemitted from the semiconductor laser element 22 for CDs at a readingpower level and the disk 12 is driven to rotate as the switch 92 isturned on. Then, a variable voltage is supplied from the voltageapplication circuit 67 to the drive circuit 68 by way of the adder 66 toforcibly drive the focus actuator section and continuously move thefocused position on the disk 12. As the focused position is moved, theS-characteristic of the focus error signal FE is observed. As shown inFIG. 19, the S-characteristic appears when the focused position islocated on the disk surface and when it is located on the pit producingsurface. The time interval from the appearance of S due to the disksurface and the one due to the pit producing surface is observed. Asseen from FIG. 19, the time interval A from the appearance of S due tothe disk surface and the one due to the pit producing surface of a DVDtype disk is shorter than the corresponding time interval B of a CD typedisk. Therefore, if the observed time interval is greater than apredetermined time threshold value, the disk is determined to be a CDtype disk. On the other hand, if the observed time interval is smallerthan the predetermined time threshold value, the disk is determined tobe DVD type disk.

If it is determined in Step S2 that the disk is of the DVD type, thedisk discriminating circuit 52 further determines if the disk 12 belongsto the first category covering single-layered DVD-ROMs and DVD-Rs or thesecond category covering double-layered DVD-ROMs, DVD-RAMs and DVD-RWs(Step S3). In the operation of Step S3, a laser beam having a wavelengthof 650 nm is emitted from the semiconductor laser element 21 for DVDs atthe reading power level and the disk 12 is driven to rotate as theswitch 92 is turned on. Then, a variable voltage is supplied from thevoltage application circuit 67 to the drive circuit 68 by way of theadder 66 to forcibly drive the focus actuator section and continuouslymove the focused position on the disk 12. As the focused position ismoved, the S-characteristic of the focus error signal FE correspondingthe pit producing surface is observed. The amplitude of the S formed bythe pit producing surface of a single-layered DVD-ROM or a DVD-R isgreater than its counterpart of a double-layered DVD-ROM, a DVD-RAM or aDVD-RW. Generally, DVD-RAMs and DVD-RWs, that are rewritable disks, aremade of a phase change material and hence show a low reflectance. Doublelayered DVD-ROMs are made semitransparent in the pit producing surfaceand hence show a low reflectance. A disk showing a low reflectance alsoshows an S having a small amplitude. Thus, the disk 12 is determined tobe either a single-layered DVD-ROM or a DVD-R of the first category whenthe amplitude of the S is smaller than a predetermined threshold value,whereas it is determined to be a double-layered DVD-ROM, a DVD-RAM or aDVD-RW of the second category.

If, in Step S3, it is determined that the disk belongs to the firstcategory and hence is either a single-layered DVD-ROM or a DVD-R, it isthen determined if the disk 12 is a single-layered DVD-ROM or a DVD-R(Step S4). In the disk discriminating operation of Step S4, a laser beamhaving a wavelength of 650 nm is emitted from the semiconductor laserelement 21 for DVDs at the reading power level and the disk 12 is drivento rotate as the switch 92 is turned on as in Step S3. However, thesupply of the variable voltage from the voltage application circuit 67is suspended. Then, the switch 65 is turned on and the focusing servocircuit 53 performs a focusing operation. Additionally, the switch 47 isturned on and the tracking servo circuit 54 performs a trackingoperation. Under this condition, it is determined if the RF signal orthe tracking error signal being read contains a groove wobble signalcomponent or not. If a groove wobble signal component is detected, thedisk is determined to be a DVD-R. If, on the other hand, no groovewobble signal component is detected, the disk is determined to be asingle-layered DVD-ROM.

As shown in FIG. 4, in case of DVD-R and DVD-RW groove wobbles 103 areformed on the pit producing surface and LPPs (land pre-pits) 104 areformed between adjacent groove wobbles 103. The LPPs 104 are used foraccurately determining the recording position on the basis of a unit ofbit and obtaining various pieces of information on the disk includingpre-addresses.

If, on the other hand, it is determined in Step S4 that the disk 12 is aDVD-R, it is then determined if the DVD-R is a general purpose DVD-R oran authoring purpose DVD-R (Step S5). More specifically, the informationof the LPPs is read out to see if the DVD-R is a general purpose DVD-Ror an authoring DVD-R after the determination of the category of thedisk in Step S4.

If, in Step S5, it is determined that the disk 12 is an authoringpurpose DVD-R, the system control circuit 1 controls the recordingoperation for the authoring purpose DVD-R. If, on the other hand, it isdetermined in Step S5 that the disk 12 is a general DVD-R, the systemcontrol circuit 1 controls the recording operation for the generalpurpose DVD-R and DVD-RW. The first tilt servo section for DVD-Rs andDVD-RWs is used for controlling the recording operation for an authoringpurpose DVD-R, a general purpose DVD-R or a DVD-RW. More specifically,the selector 88 relays the output values of the tilt correction ROM 75to the registers 96 a through 96 c in accordance with the command fromthe system control circuit 1, respectively.

If it is determined in Step S4 that the disk 12 is a single-layeredDVD-ROM, an impossible recording message is displayed on a display (notshown) (Step S6).

If it is determined in Step S3 that the disk 12 belongs to the secondcategory and may be a double-layered DVD-ROM, a DVD-RAM or a DVD-RW, itis then determined if the disk 12 is a double-layered DVD-ROM or not andhence a DVD-RW or a DVD-RAM (Step S7). In Step S7, a laser beam having awavelength of 650 nm is emitted from the semiconductor laser element 21for DVDs at the reading power level and the disk 12 is driven to rotateas the switch 92 is turned on as in Step S3. However, the supply of thevariable voltage from the voltage application circuit 67 is suspended.Then, the switch 65 is turned on and the focusing servo circuit 53performs a focusing operation. Additionally, the switch 47 is turned onand the tracking servo circuit 54 performs a tracking operation. Underthis condition, it is determined if the RF signal or the tracking errorsignal being read contains a groove wobble signal component or not. If agroove wobble signal component is detected, the disk is determined to bea DVD-RW or a DVD-RAM. If, on the other hand, no groove wobble signalcomponent is detected, the disk is determined to be a double-layeredDVD-ROM.

If it is determined in Step S7 that the disk 12 is a double-layeredDVD-ROM, the processing operation proceeds to Step S6 and an impossiblerecording message is displayed on a display screen (not shown).

If it is determined in Step S7 that the disk 12 is either a DVD-RW or aDVD-RAM, then it is determined if the disk 12 is in fact a DVD-RW or aDVD-RAM (Step S8). The operation of driving the disk player in Step S6is continued to Step S8. Thus, it is determined if the push-pull signalsuch as a tracking error signal TE contains an LPP signal componentcorresponding to an LPP or not. If an LPP signal component is detectedin the push-pull signal, the disk 12 is determined to be a DVD-RW. If,on the other hand, no LPP signal component is detected, the disk 12 isdetermined to be a DVD-RAM.

If, in Step S8, it is determined that the disk 12 is a DVD-RW, thesystem control circuit 1 controls the recording operation for a generalpurpose DVD-R or a DVD-RW. If, on the other hand, it is determined inStep S8 that the disk is a DVD-RAM, the system control circuit 1controls the recording operation for the general purpose DVD-RAM. Thesecond tilt servo section for DVD-RAMs is used for controlling therecording operation for a DVD-RAM. More specifically, the selector 88relays the output values of the tilt correction ROM 80 to the registers96 a through 96 c in response to the command from the system controlcircuit 1, respectively.

If it is determined in Step S2 that the disk is of the CD type, it isfurther determined if the disk 12 is a CD-ROM or a CD-R (Step S9). Inthe determination of Step S9, a laser beam having a wavelength of 780 nmis emitted from the semiconductor laser element 22 for CDs at thereading power level and the disk 12 is driven to rotate as the switch 92is turned on as in Step S2. However, the supply of the variable voltagefrom the voltage application circuit 67 is suspended. Then, the switch65 is turned on and the focusing servo circuit 53 performs a focusingoperation. Additionally, the switch 47 is turned on and the trackingservo circuit 54 performs a tracking operation. Under this condition, itis determined if the RF signal or the tracking error signal TE beingread contains a groove wobble signal component or not. If a groovewobble signal component is detected, the disk is determined to be aCD-R. If, on the other hand, no groove wobble signal component isdetected, the disk is determined to be a CD-ROM.

If it is determined in Step S9 that the disk 12 is a CD-ROM, theprocessing operation proceeds to Step S10 and an impossible recordingmessage is displayed on a display screen (not shown) as in Step S6.

If, on the other hand, it is determined in Step S9 that the disk is aCD-R, the system control circuit 1 controls the signal recordingoperation for the CD-R. In the recording operation for the CD-R, no tiltservo control is performed.

If it is determined in Step S1 that no recording command is issued, itis then determined if a playback command is issued from the systemcontrol circuit 1 or not (Step S11). If it is determined that theplayback command is issued, it is then determined if the disk 12 is ofthe CD type or of the DVD type (Step S12). The processing operation ofStep S12 is similar to that of Step S2.

If it is determine in Step S12 that the disk 12 is of the CD type, thesystem control circuit 1 controls playback operation for the CD typedisk. No tilt servo control operation is performed in the playbackoperation for the CD type disk.

If, on the other hand, it is determined in Step S12 that the disk is ofthe DVD type, it is then determined if the disk 12 is a DVD-RAM or not(Step S13). In the operation of Step S13, a laser beam having awavelength of 650 nm is emitted from the semiconductor laser element 21for DVDs at the reading power level and the disk 12 is driven to rotateas the switch 92 is turned on. Then, a variable voltage is supplied fromthe voltage application circuit 67 to the drive circuit 68 by way of theadder 66 to forcibly drive the focus actuator section and continuouslymove the focused position on the disk 12. As the focused position ismoved, the S-characteristic of the focus error signal FE thatcorresponds to the bit producing surface is observed. After thesuspension of the supply of the variable voltage from the voltageapplication circuit 67, the switch 65 is turned on and the focusingservo circuit 53 performs a focusing operation. Additionally, the switch47 is turned on and the tracking servo circuit 54 performs a trackingoperation. Under this condition, it is determined if the push-pullsignal such as tracking error signal TE contains an LPP signal componentcorresponding to an LPP or not. If the amplitude of the S is less than athreshold value and the push-pull signal does not contain any LPP signalcomponent, the disk is determined to be a DVD-RAM.

If it is determined in Step S13 that the disk 12 is a DVD-RAM, thesystem control circuit 1 controls playback operation for the DVD-RAM.If, on the other hand, it is determined in Step S13 that the disk 12 isa DVD type disk other than a DVD-RAM, the system control circuit 1controls playback operation for the DVD-ROM. In the playback operationfor the DVD-ROM, a tilt servo section for DVD-ROMs is used. In otherwords, the selector 88 relays the output values of the tilt correctionROM 87 to the registers 96 a through 96 c in response to the commandfrom the system control circuit 1, respectively.

For the purpose of the invention, the above described diskdiscriminating method may be replaced by some other disk discriminatingmethod such as one for detecting the contents of the TOC of the opticaldisk.

Now, the control operation of the first tilt servo section when the disk12 is a DVD-R or a DVD-RW will be discussed below.

Firstly, for controlling the operation of recording a signal on ageneral purpose DVD-R or a DVD-RW, the system control circuit 1 forciblydrives the slider 100 by means of the drive circuit 8 to move the pickup10 onto the embossed section (pre-write) section of the disk 12 andforcibly rotate the disk (Step S21). The forced rotation is typicallyrealized by feeding a spindle drive signal SPD for predeterminedrotational speed to the drive circuit 83 from an appropriate circuit(not shown). Additionally, the system control circuit 1 causes the drivecircuit 18 to drive the semiconductor laser element 21 for DVDs and makethe latter emit a laser beam having a wavelength of 650 nm at thereading power level (Step S22) and turns on the switch 92 to drive thedisk 12 to rotate under the control of the spindle servo circuit 93 forspindle rotation. Furthermore, the system control circuit 1 turns on theswitch 65 to make the focusing servo circuit 53 operate for focusing andalso the switch 47 to make the tracking servo circuit 54 operate fortracking (Step S23). Alternatively, it may be so arranged that avariable bias voltage is multiplexed with the tracking error signal todetect the bias voltage that maximizes the RF signal level.

Then, the focusing servo circuit 53 detects an optimal level of the RFsignal by means of offset regulation (Step S24). In other words, thefocusing servo circuit 53 multiplexes the focus error signal with thebias voltage from a bias circuit (not shown) and the bias voltage ismade to vary to find out the bias voltage level that maximizes the RFsignal level. Thereafter, the regions 13 a through 13 c of the liquidcrystal panel 13 are driven by the tilt servo system to detect theoptimum RF signal level (Step S25). More specifically, the correctionvalues of the regions 13 a through 13 c that correspond to therespective addresses of the tilt correction ROM 75 are read out andsequentially fed to the respective registers 96 a through 96 c by way ofthe selector 88. Then, the PWM sections 97 a through 97 c are drive togenerate tilt drive signals TID in order to drive the respective regions13 a through 13 c of the liquid crystal panel 13 by way of the drivecircuit 28 so that the address value or the three correction values thatmaximize the RF signal level may be detected.

When the optimum RF signal level is detected, the level of the outputsignal of the adder 69 is stored in the reference memory 71 as referencesignal (Step S26). The changeover switch 72 relays the output signal ofthe adder 69 to the reference memory 71 while the processing operationof Step S26 is being carrier out but, when the operation of Step S26 isover, it is switched to relay the output signal of the adder 69, whichis a push-pull offset signal, to the subtracter 72. Then, it performs atentative writing operation in the PCA area of the disk to determine anoptimal recording power level of the laser beam (Step S27). Thereafter,it starts a recording operation (Step S28) and drives the regions 13 athrough 13 c of the liquid crystal panel 13 according to the tilt signalcomputationally determined by the first tilt servo section for DVD-Rsand DVD-RWs (Step S29). More specifically, level difference signalrepresenting the level difference between the output signal of the adder69 and the reference signal stored in the reference memory 71 isproduced from the subtracter 72 and supplied to the A/D converter 74 byway of the low pass filter 73 as tilt error signal. The output signal ofthe A/D converter 74 represents addresses in the tilt correction ROM 75and three correction values corresponding to the addresses are read outof the tilt correction ROM 75. The three correction values are then fedto the respective registers 96 a through 96 c by way of the selector 88so that the PWM sections 97 a through 97 c generate corresponding tiltdrive signals TID, which are then used by the drive circuit 28 to drivethe respective regions 13 a through 13 c of the liquid crystal panel 13.The processing operation of Step S29 continues until the recordingoperation ends.

For controlling the recording operation on the authoring purpose DVD-R,the system control circuit 1 forcibly drives the slider 100 by means ofthe drive circuit 8 so as to move the pickup 10 to the PCA area of thedisk 12 and forcibly rotate the disk 12 (Step S31). Then, it issues acommand for driving the semiconductor laser element 21 for DVDs to emita laser beam having a wavelength of 650 nm at the recording power level(Step S32). The system control circuit 1 then turns on the switch 92 todrive the disk 12 to rotate under the control of the spindle servocircuit 93 for spindle rotation. Furthermore, the system control circuit1 turns on the switch 65 to make the focusing servo circuit 53 operatefor focusing and also the switch 47 to make the tracking servo circuit54 operate for tracking (Step S33). Then, it performs a tentativewriting operation in the PCA area of the disk to determine an optimalrecording power level of the laser beam (Step S34). Subsequently, itchanges the recording power level stepwise to preliminarily determine anoptimum recording power level.

After carrying out Step S27, the system control circuit 1 issues acommand for driving the semiconductor laser element 21 for DVDs to emita laser beam having a wavelength of 650 nm at the reading power level tothe tentatively written pit (Step S35) and causes the focusing servocircuit 53 to detect an optimum RF signal level by means of offsetregulation (Step S36). More specifically, the focusing servo circuit 53multiplexes the focus error signal with the bias voltage from a biascircuit (not shown) and the bias voltage is made to vary to find out thebias voltage level that maximizes the RF signal level. It may be soarranged that the tracking servo circuit 54 also detects an optimum RFsignal level also by means of offset regulation at the same time.Thereafter, the regions 13 a through 13 c of the liquid crystal panel 13are driven by the tilt servo system to detect the optimum RF signallevel (Step S37). More specifically, the correction values of theregions 13 a through 13 c that correspond to the respective addresses ofthe tilt correction ROM 75 are read out and sequentially fed to therespective registers 96 a through 96 c by way of the selector 88. Then,the PWM sections 97 a through 97 c are driven to generate tilt drivesignals TID in order to drive the respective regions 13 a through 13 cof the liquid crystal panel 13 by way of the drive circuit 28 so thatthe address value or the three correction values that maximize the RFsignal level may be detected.

When the optimum RF signal level is detected, the level of the outputsignal of the adder 69 is stored in the reference memory 71 as referencesignal (Step S38). The changeover switch 70 relays the output signal ofthe adder 69 to the reference memory 71 while the processing operationof Step S38 is being carrier out but, when the operation of Step S38 isover, it is switched to relay the output signal of the adder 69, whichis a push-pull offset signal, to the subtracter 72. Then, it performs atentative writing operation in the PCA area of the disk to determine anoptimal recording power level of the laser beam (Step S39). Thereafter,it starts a recording operation (Step S40) and drives the regions 13 athrough 13 c of the liquid crystal panel 13 according to the tilt errorsignal computationally determined by the first tilt servo section forDVD-Rs and DVD-RWs (Step S41). The processing operation of Step S41continues until the recording operation ends.

The user can pre-write management information or the like on anauthoring purpose DVD-R. FIG. 22 is a flow chart of a tilt servo controloperation of the first tilt servo section that can be used for anauthoring purpose DVD-R when the use of the pre-write feature isinvolved. Referring to FIG. 22, after carrying out Steps S31 throughS34, the system control circuit 1 forcibly drives the slider 100 bymeans of the drive circuit 8 to move the pickup 10 onto the pre-writesection for a pre-write operation (which corresponds to the pre-writesection of a general purpose DVD) of the disk 12 (Step S61) and issues acommand for driving the semiconductor laser element 21 for DVDs to emita laser beam having a wavelength of 650 nm at a preliminarily determinedoptimum recording power level (Step S62) and write a signal in thepre-write section of the disk with the preliminarily determined optimumrecording power level (Step S63). After Step S63, the system controlcircuit 1 issues a command for driving the semiconductor laser element21 for DVDs to emit a laser beam having a 650 nm at the reading powerlevel (Step S64) and then proceeds to Step S36, where it causes thefocusing servo circuit 53 to detects an optimum RF signal level by meansof offset regulation. Thereafter, the system control circuit 1 performsSteps S37 through S41. Note that the system control circuit 1 forciblydrives the slider 100 by means of the drive circuit 8 to move the pickup10 onto the PCA area of the disk 12 before executing Step S39 (StepS65).

While a tilt servo control device including a liquid crystal panel 13 isused as tilt angle adjusting means for compensating the tilt angle inthe above embodiment, an actuator for mechanically regulating theinclination of the pickup or the objective lens relative to the opticaldisk may be used to replace the tilt servo control device. Then, theactuator will be driven in accordance with a tilt error signal.

As described above in detail, according to the invention, there areprovided a tilt servo control device and a tilt servo control methodthat can be used with mint optical recording media bearing no writteninformation for the purpose of tilt servo control operations.

The present invention can be applied to not only an optical disk playerwith a recording function but also an optical disk player without arecording function.

According to the present invention, a tilt servo control can be properlyperformed for an unwritten optical disk into which any information isnot written yet.

This application is based on Japanese Patent Application No. 2000-367304which is hereby incorporated by reference.

1. A tilt servo control device of an information recording/reproducingapparatus for recording information on and reproducing information froman optical recording medium, said apparatus comprising an optical systemfor leading a laser beam emitted from a light source to a recordingsurface of the optical recording medium and a laser beam reflected bythe recording surface of the recording medium to a split type photodetector and a read signal generator for generating a read signal inaccordance with an output signal of the photo detector, said devicecomprising: a push-pull component generator for generating a push-pullcomponent signal including a differential component of output signals ofrespective split portions of said photo detector; a tilt error signalgenerator for generating a tilt error signal representing a tilt anglebetween a normal to the recording surface of said optical recordingmedium at a position of said laser beam irradiating the recordingsurface and an optical axis of said laser beam based on said push-pullcomponent signal; a tilt angle adjuster for adjusting said tilt angle; adriver for driving said tilt angle adjuster so as to reduce said tilterror signal; and a holder for holding, as a reference signal, a signalobtained by smoothing said push-pull component signal corresponding tosaid read signal showing a desired level at a predetermined area of saidoptical recording medium: wherein said tilt error signal generatorgenerates the tilt error signal in accordance with the differencebetween said signal obtained by smoothing said push-pull componentsignal and said reference signal.
 2. A tilt servo control deviceaccording to claim 1, wherein said photo detector has a main lightreceiving section which is divided into at least two portions and twosub light receiving sections between which said main light receivingsection is inserted, which are respectively divided into two portions,each of said main light receiving section and said sub light receivingsections being arranged so that the divided line thereof is parallelwith a tangential direction of a track of said optical recording medium,and generating an output signal for each divided portion; said apparatuscomprising: a main push-pull signal generator for generating a mainpush-pull signal representing the differential component of the outputsignals of said main light receiving section; a sub push-pull generatorfor generating first and second sub push-pull signals representing thedifferential components of the output signals of said two sub lightreceiving sections; and an adder for adding said first and second subpush-pull signals and multiplying the sum of the addition by apredetermined coefficient to generate a sub push-pull sum signal;wherein said push-pull component signal is one of a main/sub push-pullsum signal obtained by adding said main push-pull signal and said subpush-pull sum signal, said main push-pull signal and said sub push-pullsum signal.
 3. A tilt servo control device according to claim 1, furthercomprising a setting device for setting a recording power level of thelaser beam output from said light source by carrying out a tentativewriting operation in a power calibration area before an actualinformation recording operation after being holding said referencesignal by said holder.
 4. A tilt servo control device according to claim1, wherein said predetermined area of said optical recording medium isan embossed area, a pre-write area or a power calibration area having apit formed by a tentative writing operation.
 5. A tilt servo controldevice according to claim 4, wherein said predetermined area is theembossed area when said optical recording medium is a DVD-RW.
 6. A tiltservo control device according to claim 4, wherein said predeterminedarea is the pre-write area when said optical recording medium is ageneral purpose DVD-R.
 7. A tilt servo control device according to claim4, wherein said predetermined area is the power calibration area whensaid optical recording medium is an authoring purpose DVD-R.
 8. A tiltservo control device according to claim 1, wherein said read signal iscontrolled to show a desired level by means of offset regulation of afocusing servo system and the drive against said tilt angle adjuster bysaid driver.
 9. A tilt servo control device according to claim 1,wherein said tilt angle adjuster includes a liquid crystal panel havinga plurality of regions arranged in said optical system, and said driverdrives each of said plurality of regions of said liquid crystal panel inaccordance with said tilt error signal.
 10. A tilt servo control deviceaccording to claim 1, wherein said tilt error signal generator includesa subtracter for computing a difference between said push-pull componentsignal and said reference signal.
 11. A tilt servo control method of aninformation recording/reproducing apparatus for recording information onand reproducing information from an optical recording medium, saidapparatus comprising an optical system for leading a laser beam emittedfrom a light source to a recording surface of the optical recordingmedium and a laser beam reflected by the recording surface of therecording medium to a split type photo detector and a read signalgenerator for generating a read signal in accordance with an outputsignal of the photo detector, said method comprising the steps of:generating a push-pull component signal including a differentialcomponent of output signals of respective split portions of said photodetector; generating a tilt error signal representing a tilt anglebetween a normal to the recording surface of said optical recordingmedium at a position of said laser beam irradiating the recordingsurface and an optical axis of said laser beam based on said push-pullcomponent signal; driving a tilt angle adjuster for adjusting said tiltangle so as to reduce said tilt error signal; and holding, as areference signal, a signal obtained by smoothing said push-pullcomponent signal corresponding to said read signal showing a desiredlevel at a predetermined area of said optical recording medium, whereinthe tilt error signal is generated in accordance with the differencebetween said signal obtained by smoothing said push-pull componentsignal and said reference signal.